1/MAO. It is an indication of a rampant site epimerization setting in faster and
faster with increasing polymerization temperature. The concentrations of the site
epimerization related rrmr pentads in the syndiotactic polymers produced with the
5/MAO catalyst system have, however, almost triple the size of corresponding rrmr
pentads found in polymers produced with 1/MAO. The enantioselectivity related
(rmmr) pentad concentrations in s-PP chains produced at the same polymerization
temperature with both catalyst systems are less different in size and do not change
as dramatically with the change in polymerization temperature, within the range
tested (see data on related pentads in Table 4).
The observed general similarities in catalytic behavior between 5/MAO and
1/MAO catalyst systems corroborate the molecular structural resemblance of 1
and 5 but are in contradiction with the displayed discrepancies regarding larger
polydispersity, higher molecular weight, and lower stereoregularity. The complete
interpretation of the polymerization data can, however, be given in an elegant and
convincing way, as least with respect to larger polydispersity, by close inspection of
the single-crystal X-ray structure and associated data [34, 113] (similar results have
been reported by Kim et al. [114]) and by inspecting more closely the impact of the
bridge as the potential source of behavioral dissimilarity.
Let us first start the discussion with the polydispersity and focus on the reasons
for a relatively large molecular weight distribution (up to 4.7) observed for s-PP
polymers produced with 5/MAO catalyst systems. It follows from the theory [115]
that for polymers produced with single-site type catalysts the expected molecular
weight distributions should have a value of 2. Accordingly, values of 2 (or close to 2)
have been measured generally for all polymers produced in homogeneous olefin
polymerization catalysis with activated metallocenes under controlled polymerization conditions. Large deviations from this value are generally indicative of the
presence of diffusion phenomena or temperature variations during the polymerization process. In the absence of the said factors, broader molecular weight distribution
reflect, unambiguously, the presence of more than one population of active
species in the polymerization medium. Closer inspection of the crystal structure
data for 5 confirms this fact and provides the clue for this apparently rather unusual
phenomenon.
The in-depth examinations of the X-ray data for 5 by Atwood and colleagues
[113] revealed that the unit cell of the crystalline lattice of 5 accommodates two
types of molecules, i.e., two different conformers. The formation of two conformers
arises from the fact that the ligand structure of complex 5 contains a two-carbon
atom unit in its ethano-based bridge that has a fluxional character. The ethano
(–CH 2 –CH 2 –) group can assume two different spatial arrangements with respect to
the aromatic rings, being able to oscillate about the center of inertia lying mid-way
between the two CH 2 groups. These arrangements give rise to two energetically
identical, stable, but different conformers, δ and λ, whose structures are presented
in Fig. 11. The two independent molecules populate equally the unit cells of the
crystalline lattice in the solid state.
These conformers, as neutral molecules, interconvert very quickly in solution at
room temperature and are indistinguishable (in a temperature range measured from
À80
C to 90
C) in the
1 H NMR timescale [34]. The
1 H NMR spectrum of 5
62
A. Razavi
faster with increasing polymerization temperature. The concentrations of the site
epimerization related rrmr pentads in the syndiotactic polymers produced with the
5/MAO catalyst system have, however, almost triple the size of corresponding rrmr
pentads found in polymers produced with 1/MAO. The enantioselectivity related
(rmmr) pentad concentrations in s-PP chains produced at the same polymerization
temperature with both catalyst systems are less different in size and do not change
as dramatically with the change in polymerization temperature, within the range
tested (see data on related pentads in Table 4).
The observed general similarities in catalytic behavior between 5/MAO and
1/MAO catalyst systems corroborate the molecular structural resemblance of 1
and 5 but are in contradiction with the displayed discrepancies regarding larger
polydispersity, higher molecular weight, and lower stereoregularity. The complete
interpretation of the polymerization data can, however, be given in an elegant and
convincing way, as least with respect to larger polydispersity, by close inspection of
the single-crystal X-ray structure and associated data [34, 113] (similar results have
been reported by Kim et al. [114]) and by inspecting more closely the impact of the
bridge as the potential source of behavioral dissimilarity.
Let us first start the discussion with the polydispersity and focus on the reasons
for a relatively large molecular weight distribution (up to 4.7) observed for s-PP
polymers produced with 5/MAO catalyst systems. It follows from the theory [115]
that for polymers produced with single-site type catalysts the expected molecular
weight distributions should have a value of 2. Accordingly, values of 2 (or close to 2)
have been measured generally for all polymers produced in homogeneous olefin
polymerization catalysis with activated metallocenes under controlled polymerization conditions. Large deviations from this value are generally indicative of the
presence of diffusion phenomena or temperature variations during the polymerization process. In the absence of the said factors, broader molecular weight distribution
reflect, unambiguously, the presence of more than one population of active
species in the polymerization medium. Closer inspection of the crystal structure
data for 5 confirms this fact and provides the clue for this apparently rather unusual
phenomenon.
The in-depth examinations of the X-ray data for 5 by Atwood and colleagues
[113] revealed that the unit cell of the crystalline lattice of 5 accommodates two
types of molecules, i.e., two different conformers. The formation of two conformers
arises from the fact that the ligand structure of complex 5 contains a two-carbon
atom unit in its ethano-based bridge that has a fluxional character. The ethano
(–CH 2 –CH 2 –) group can assume two different spatial arrangements with respect to
the aromatic rings, being able to oscillate about the center of inertia lying mid-way
between the two CH 2 groups. These arrangements give rise to two energetically
identical, stable, but different conformers, δ and λ, whose structures are presented
in Fig. 11. The two independent molecules populate equally the unit cells of the
crystalline lattice in the solid state.
These conformers, as neutral molecules, interconvert very quickly in solution at
room temperature and are indistinguishable (in a temperature range measured from
À80
C to 90
C) in the
1 H NMR timescale [34]. The
1 H NMR spectrum of 5
62
A. Razavi